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What is Merkel cell carcinoma?

Merkel cell carcinoma (MCC) is an uncommon type of skin cancer. It can grow quickly and can be hard to treat if it spreads beyond the skin. To understand Merkel cell carcinoma, it helps to know about the skin and its parts.

Normal skin

The skin is the largest organ in your body. It does many different things, such as:

Epidermis

The top layer of skin is the epidermis. The epidermis is very thin, averaging only about 1/100 of an inch thick. It protects the deeper layers of skin and the organs of the body from the environment.

The cells near the outer surface of the epidermis are called squamous cells because of their flat shape.

The lowest part of the epidermis is called the basal layer. Most of the cells here are called basal cells, but this layer also contains Merkel cells, the cells that can develop into Merkel cell carcinoma.

Merkel cells are very close to nerve endings in the skin. They help us sense light touch, which allows us to do things like feel the fine details on an object’s surface. Merkel cells are thought to be a type of neuroendocrine cell, because they share some features with nerve cells and hormone-making cells.

The epidermis is separated from the deeper layers of skin by the basement membrane. As a skin cancer becomes more advanced, it can grow through this barrier and into the deeper layers of the skin.

Dermis

The middle layer of the skin, called the dermis, is much thicker than the epidermis. It contains hair follicles, sweat glands, blood vessels, and nerves. These are held in place by a protein called collagen, which gives the skin its elasticity and strength.

Subcutis

The deepest layer of the skin is called the subcutis. It contains a network of collagen and fat cells. The subcutis helps the body conserve heat and has a shock-absorbing effect that helps protect the body’s organs from injury.

Merkel cell carcinoma

Merkel cell carcinoma is an uncommon type of skin cancer that starts when Merkel cells grow out of control. Because Merkel cells are a type of neuroendocrine cell, MCC is also sometimes called neuroendocrine carcinoma of the skin. Another name for MCC is trabecular carcinoma (or trabecular cancer).

MCC is much less common than most other types of skin cancer (see below), but it’s one of the most dangerous types. It’s much more likely than common skin cancers to spread to other parts of the body if not caught early, and it can be very hard to treat if it has spread.

These cancers most often start on sun-exposed parts of the skin, such as the face (the most common site), neck, and arms. But MCC can start anywhere on the body. Merkel cell tumors usually appear as firm, pink, red, or purple lumps or bumps on the skin. They are not usually painful, but they can grow quickly and can sometimes open up as ulcers or sores (see “Signs and symptoms of Merkel cell carcinoma”).

While nearly all MCCs start on the skin, a very small portion start in other parts of the body, such as inside the nose or esophagus.

Other types of skin cancer

Basal and squamous cell carcinomas

These are by far the most common skin cancers. They rarely spread to other parts of the body, and can usually be cured with surgery. For more information on these cancers, see our document Skin Cancer: Basal and Squamous Cell.

Melanomas

These cancers develop from melanocytes, the pigment-making cells of the skin. They are much less common that basal and squamous cell cancers, but they are much more likely to spread and be life-threatening if not caught at an early stage. Melanoma is discussed in our document Melanoma Skin Cancer.